1. Field of the Invention
This invention relates to a continuous aeration biophysical step for treating wastewater followed by a batch clarification step to complete the treatment process.
2. Related Prior Art
The quantity of pollutants in wastewater is commonly determined by measuring the amount of dissolved oxygen required to biologically decompose the waste organic matter in the polluted water. This measurement, called biochemical oxygen demand (BOD), provides an index of the organic pollution of water. Some organic contaminants, such as chlorinated aromatic compounds, are not amenable to conventional biological decomposition and tests such as chemical oxygen demand (COD) and total organic carbon (TOC) have been employed to measure the concentration of these compounds.
Various aerobic wastewater treatment processes are in use which are classified either as continuous flow or batch flow in their mode of operation. Each process has advantages as well as shortcomings with respect to performance, stability and adaptability to changing wastewater characteristics.
The sequencing batch reactor (SBR) or batch mode of wastewater treatment is advanced as a simple, single tank process comprising a timed sequence of equalization, aeration and liquid/solids separation as opposed to continuous flow processes which employ a space sequence of the above functions. The batch mode offers greater flexibility of operation and control in that it is easier to adjust a time period than to change a space to redistribute process functions.
One of the advantages of the SBR process is that settling takes place under quiescent conditions in a basin under near ideal settling conditions. This also assures that the microorganisms remain in the system since the mixed liquor solids are not returned from a separate sedimentation basin. However, fluctuations in wastewater characteristics or other process upsets can result in poor settling of mixed liquor solids from treated wastewater, culminating in passage of solids from the system with the effluent and a loss of process performance. Alternatively, decanting a reduced volume of effluent per cycle decreases the volumetric throughput of the SBR system.
Nicol, U.S. Pat. No. 3,366,242 disclose a continuous flow system for biological treatment of wastewater containing a first aeration/predigestion chamber fitted with an air lift pump to transfer waste to a settling chamber for solids removal and effluent discharge. Settled sludge is recycled to the first chamber.
Nicol also discloses treatment systems using two aeration chambers in series. In U.S. Pat. No. 3,522,881 the second aeration chamber functions alternately as an aeration chamber and a settling chamber with effluent removed therefrom. In U.S. Pat. No. 3,524,547 influent wastewater flow alternates between the first and second aeration chambers and the chamber not receiving feed receives flow from the other chamber and operates as a settling chamber without aeration.
Koulovatos et al., U.S. Pat. No. 3,679,053 discloses two aeration tanks operated in series with the second tank acting as a settling basin without aeration for a portion of time. The effluent from the second aeration/settling tank flows to another settling tank for further clarification. Solids from the second settling tank are returned to the first aeration basin. Similar flow schemes are described by Gensman et al., U.S. Pat. No. 3,746,638 and in GB No. 1,549,400 where solids from both settling tanks are recycled to the first aeration tank.
A particularly useful process for removing pollutants from wastewater employs a mixture of bacteria and powdered activated carbon in a treatment zone. This process, called the PACT.RTM. treatment system, is disclosed in Hutton et al., U.S. Pat. Nos. 3,904,518 and 4,069,148. The PACT treatment system operates as a continuous flow process with an aeration basin followed by a separate clarifier to separate biologically active solids and carbon from the treated wastewater, with the settled sludge returned to the aeration basin.
A slightly different biophysical treatment process is described by McShane et al. in "Biophysical Treatment of Landfill Leachate Containing Organic Compounds", Proceedings of Industrial Waste Conference, 1986 (Pub. 1987), 41st, 167-77. In this process a biological batch reactor is used with powdered activated carbon and the system is operated in the "fill and draw" or SBR mode. A similar scheme for treatment of leachate is disclosed in Ying et al., U.S. Pat. No. 4,623,464 in which an SBR is operated with both biologically active solids and carbon present to treat PCB and dioxin-containing leachate.